Guill was a male Mari merchant who dealt officially in technical equipment such as resonator coils, but also in violent thoughts, which were outlawed on his homeworld. He claimed to have several children.
By 2374, he ran the black market trade of these violent thoughts. When the USS Voyager spent a few days in orbit of his homeworld, Guill, intrigued by the possibilities offered by the thoughts of Voyager''s more violent crewmembers such as B'Elanna Torres, set up an incident where Torres would be angered long enough for him to acquire some of her more violent thoughts. He instructed Frane to bump into her, so Torres would have an emotional reaction. He then offered to help Torres, touched her and telepathically stole her violent thoughts.
He then sold those thoughts to an elderly woman named Tembit, thereby indirectly causing at least one death and framing Torres as the source, resulting in her being scheduled for a potentially brain-damaging engramatic purge.
Tuvok, after a Vulcan mind meld with Torres, saw the probing done by Guill, and investigated him. Under the pretense of selling Guill more violent thoughts, he mind melded with him, but instead of transferring violent images, he caused Guill great pain, the sheer intensity of a Vulcan's suppressed darker emotions proving too much for Guill to handle, forcing him to confess. He was then arrested by Chief Examiner Nimira of the Mari Constabulary. (VOY: "Random Thoughts")